In a wireless communication system, a transmitter and a receiver may obtain a higher rate of data transmission by adopting multiple antennas in a spatial multiplexing manner. With respect to a common spatial multiplexing method, an enhancement technology, in which a receiver may feed back channel information to a transmitter and the transmitter may employ some transmission precoding technologies according to the obtained channel information, may be adopted to greatly improve the transmission performance. In a single-user Multi-input Multi-output (MIMO), channel characteristic vector information may be directly used for precoding. In a multi-user MIMO, more accurate channel information may be needed.
In a Long Term Evolution (LTE) plan, channel information is mainly fed back using a relatively simple feedback method with a single codebook. The performance of the transmission precoding technology of the MIMO may be more reliant on the accuracy of codebook feedback.
Here, a basic principle of channel information quantized feedback based on the codebook is briefly described as follows.
It is assumed that the capacity of a limited feedback channel is B bps/Hz, then the number of available code words may be N=2B. A characteristic vector space of a channel matrix may be quantized to form a codebook space ={F1, F2L FN}. The transmitter and the receiver may both store the codebook or generate the codebook in real time. Herein, the codebook at the transmitter and the receiver is the same. According to the channel matrix H obtained by the receiver, the receiver may select a code word {circumflex over (F)} most matched with the channel from the codebook space  according to a certain criterion and may feed a code word serial number i back to the transmitter. Here, the code word serial number is called as a Precoding Matrix Indicator (PMI). The transmitter may find a corresponding precoding code word {circumflex over (F)} according to the code word serial number i, thereby obtaining the channel information. The precoding code word {circumflex over (F)} represents the characteristic vector information of the channel.
Generally speaking, the codebook space  may further be divided into codebooks corresponding to multiple ranks. Under each rank, there may be multiple corresponding code words to quantize the precoding matrix formed by channel characteristic vectors under this rank. The number of the ranks of the channel is the same as that of nonzero characteristic vectors, so in general, when the number of ranks is N, the number of columns in each code word is N. Hence, the codebook  may be divided into multiple sub-codebooks in terms of different ranks, as shown in a Table A1.
TABLE A1  Number of layers υ (Rank)12. . .N  1  2. . .  NA set of code wordA set of code wordA set of code wordvectors having onevectors having twovectors having Ncolumncolumnscolumns
In the table, 1 represents a set of code word vectors having one column, 2 represents a set of code word vectors having two columns, and N represents a set of code word vectors having N columns.
Herein, when Rank>1, the code words needing to be stored are all in a matrix form. In an LTE protocol, the codebook is fed back by this quantized codebook feedback method. The codebook for downlink four transmitting antennas in an LTE Rel-8 version is as shown in a Table A2. As a matter of fact, the precoding codebook and the channel information quantized codebook in the LTE may have the same meaning. Hereinafter, for simplicity, a vector may also be regarded as a one-dimensional matrix.
TABLE A2CodewordTotal number of layers υ (RI)indexun12340u0 = [1 −1 −1 −1]TW0{1}W0{14}/{square root over (2)}W0{124}/{square root over (3)}W0{1234}/{square root over (2)}1u1 = [1 −j 1 j]TW1{1}W1{12}/{square root over (2)}W1{123}/{square root over (3)}W1{1234}/{square root over (2)}2u2 = [1 1 −1 1]TW2{1}W2{12}/{square root over (2)}W2{123}/{square root over (3)}W2{3214}/{square root over (2)}3u3 = [1 j 1 −j]TW3{1}W3{12}/{square root over (2)}W3{123}/{square root over (3)}W3{3214}/{square root over (2)}4u4 = [1 (−1 − j)/{square root over (2)} −j (1 − j)/{square root over (2)}]TW4{1}W4{14}/{square root over (2)}W4{124}/{square root over (3)}W4{1234}/{square root over (2)}5u5 = [1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]TW5{1}W5{14}/{square root over (2)}W5{124}/{square root over (3)}W5{1234}/{square root over (2)}6u6 = [1 (1 + j){square root over (2)} −j (−1 + j)/{square root over (2)}]TW6{1}W6{13}/{square root over (2)}W6{134}/{square root over (3)}W6{1324}/{square root over (2)}7u7 = [1 (−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]TW7{1}W7{13}/{square root over (2)}W7{134}/{square root over (3)}W7{1324}/{square root over (2)}8u8 = [1 −1 1 1]TW8{1}W8{12}/{square root over (2)}W8{124}/{square root over (3)}W8{1234}/{square root over (2)}9u9 = [1 −j −1 − j]TW9{1}W9{14}/{square root over (2)}W9{134}/{square root over (3)}W9{1234}/{square root over (2)}10u10 = [1 1 1 −1]TW10{1}W10{13}/{square root over (2)}W10{123}/{square root over (3)}W10{1324}/{square root over (2)}11u11 = [1 j −1 j]TW11{1}W11{13}/{square root over (2)}W11{134}/{square root over (3)}W11{1324}/{square root over (2)}12u12 = [1 −1 −1 1]TW12{1}W12{12}/{square root over (2)}W12{123}/{square root over (3)}W12{1234}/{square root over (2)}13u13 = [1 −1 1 −1]TW13{1}W13{13}/{square root over (2)}W13{123}/{square root over (3)}W13{1324}/{square root over (2)}14u14 = [1 1 −1 −1]TW14{1}W14{13}/{square root over (2)}W14{123}/{square root over (3)}W14{3214}/{square root over (2)}15u15 = [1 1 1 1]TW15{1}W15{12}/{square root over (2)}W15{123}/{square root over (3)}W15{1234}/{square root over (2)}
Herein, Wn=I−2ununH/unHun, I is a unit matrix, Wk(j) represents a jth column vector of a matrix Wk(j). Wk(j1, j2, . . . jn) represents a matrix formed by j1, j2, . . . , jnth columns of the matrix Wk.
The code words under rank 1 for Rel-10 LTE downlink 8Tx are as shown in a Table A3, and the codebook under rank 2 is as shown in a Table A4.
TABLE A3i2i101 2 3 4 5 6 70-15W2i1,0(1)W2i1,1(1)W2i1,2(1)W2i1,3(1)W2i1+1,0(1)W2i1+1,1(1)W2i1+1,2(1)W2i1+1,3(1)i2i1891011121314150-15W2i1+2,0(1)W2i1+2,1(1)W2i1+2,2(1)W2i1+2,3(1)W2i1+3,0(1)W2i1+3,1(1)W2i1+3,2(1)W2i1+3,3(1)       where    ⁢                  ⁢          W              m        ,        n                    (        1        )              =            1              8              ⁡          [                                                  v              m                                                                                          φ                n                            ⁢                              v                m                                                        ]      
TABLE A4i2i1 0 1 2 30-15W2i1,2i1,0(2)W2i1,2i1,1(2)W2i1+1,2i1+1,0(2)W2i1+1,2i1+1,1(2)i2i1 4 5 6 70-15W2i1+2,2i1+2,0(2)W2i1+2,2i1+2,1(2)W2i1+3,2i1+3,0(2)W2i1+3,2i1+3,1(2)i2i1 8 910110-15W2i1,2i1+1,0(2)W2i1,2i1+1,1(2)W2i1+1,2i1+2,0(2)W2i1+1,2i1+2,1(2)i2i1121314150-15W2i1,2i1+3,0(2)W2i1,2i1+3,1(2)W2i1+1,2i1+3,0(2)W2i1+1,2i1+3,1(2)       where    ⁢                  ⁢          W              m        ,                  m          ′                ,        n                    (        2        )              =            1      4        ⁡          [                                                  v              m                                                          v                              m                ′                                                                                                        φ                n                            ⁢                              v                m                                                                                        -                                  φ                  n                                            ⁢                              v                                  m                  ′                                                                        ]      
Where:φn=ejπn/2 vm=[1ej2πm/32ej4πm/32ej6πm/32]T  (1)
Table A5 to Table A10 respectively show the codebook under ranks 3 to 8 for the 8Tx.
TABLE A5i2i1 0 1 2 30-3W8i1,8i1,8i1+8(3)W8i1+8,8i1,8i1+8(3){tilde over (W)}8i1,8i1+8,8i1+8(3){tilde over (W)}8i1+8,8i1,8i1(3)i2i1 4 5 6 70-38i1+2,8i1+2,8i1+10(3)W8i1+10,8i1+2,8i1+10(3){tilde over (W)}8i1+2,8i1+10,8i1+10(3){tilde over (W)}8i1+10,8i1+2,8i1+2(3)i2i1 8 910110-3W8i1+4,8i1+4,8i1+12(3)W8i1+12,8i1+4,8i1+12(3){tilde over (W)}8i1+4,8i1+12,8i1+12(3){tilde over (W)}8i1+12,8i1+4,8i1+4(3)i2i1121314150-3W8i1+6,8i1+6,8i1+14(3)W8i1+14,8i1+6,8i1+14(3){tilde over (W)}8i1+6,8i1+14,8i1+14(3){tilde over (W)}8i1+14,8i1+6,8i1+6(3)             where      ⁢                          ⁢              W                  m          ,                      m            ′                    ,                      m            ″                                    (          3          )                      =                  1                  24                    ⁡              [                                                            v                m                                                                    v                                  m                  ′                                                                                    v                                  m                  ″                                                                                                        v                m                                                                    -                                  v                                      m                    ′                                                                                                      -                                  v                                      m                    ″                                                                                      ]              ,             W      ~              m      ,              m        ′            ,              m        ″                    (      3      )        =            1              24              ⁡          [                                                  v              m                                                          v                              m                ′                                                                        v                              m                ″                                                                                        v              m                                                          v                              m                ′                                                                        -                              v                                  m                  ″                                                                        ]      
TABLE A6i2i101230-3W8i1,8i1,+8,0(4)W8i1,8i1+8,1(4)W8i1+2,8i1+10,0(4)W8i1+2,8i1+10,1(4)i2i145670-3W8i1+4,8i1+12,0(4)W8i+4,8i1+12,1(4)W8i1+6,8i1+14,0(4)W8i1+6,8i1+14,1(4)       where    ⁢                  ⁢          W              m        ,                  m          ′                ,        n                    (        4        )              =            1              32              ⁡          [                                                  v              m                                                          v                              m                ′                                                                        v              m                                                          v                              m                ′                                                                                                        φ                n                            ⁢                              v                m                                                                                        φ                n                            ⁢                              v                                  m                  ′                                                                                                        -                                  φ                  n                                            ⁢                              v                m                                                                                        -                                  φ                  n                                            ⁢                              v                                  m                  ′                                                                        ]      
TABLE A7i2i100-3      W          i      1              (      5      )        =            1              40              ⁡          [                                                  v                              2                ⁢                                  i                  1                                                                                        v                              2                ⁢                                  i                  1                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                8                                                                        v                                                2                  ⁢                                      i                    1                                                  +                8                                                                        v                                                2                  ⁢                                      i                    1                                                  +                16                                                                                        v                              2                ⁢                                  i                  1                                                                                        -                              v                                  2                  ⁢                                      i                    1                                                                                                          v                                                2                  ⁢                                      i                    1                                                  +                8                                                                        -                              v                                                      2                    ⁢                                          i                      1                                                        +                  8                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                16                                                        ]      
TABLE A8i2i100-3      W          i      1              (      6      )        =            1              48              ⁡          [                                                  v                              2                ⁢                                  i                  1                                                                                        v                              2                ⁢                                  i                  1                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                8                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                8                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                16                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                16                                                                                        v                              2                ⁢                                  i                  1                                                                                        -                              v                                  2                  ⁢                                      i                    1                                                                                                          v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                8                                                                        -                              v                                                      2                    ⁢                                          i                      1                                                        +                                                                          ⁢                  8                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                16                                                                        -                              v                                                      2                    ⁢                                          i                      1                                                        +                                                                          ⁢                  16                                                                        ]      
TABLE A9i2i100-3      W          i      1              (      7      )        =            1              56              ⁡          [                                                  v                              2                ⁢                                  i                  1                                                                                        v                              2                ⁢                                  i                  1                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                8                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                8                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                16                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                16                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                24                                                                                        v                              2                ⁢                                  i                  1                                                                                        -                              v                                  2                  ⁢                                      i                    1                                                                                                          v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                8                                                                        -                              v                                                      2                    ⁢                                          i                      1                                                        +                                                                          ⁢                  8                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                16                                                                        -                              v                                                      2                    ⁢                                          i                      1                                                        +                                                                          ⁢                  16                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                24                                                        ]      
TABLE A10i2i100      W          i      1              (      8      )        =            1      8        ⁡          [                                                  v                              2                ⁢                                  i                  1                                                                                        v                              2                ⁢                                  i                  1                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                8                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                8                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                16                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                16                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                24                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                24                                                                                        v                              2                ⁢                                  i                  1                                                                                        -                              v                                  2                  ⁢                                      i                    1                                                                                                          v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                8                                                                        -                              v                                                      2                    ⁢                                          i                      1                                                        +                                                                          ⁢                  8                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                16                                                                        -                              v                                                      2                    ⁢                                          i                      1                                                        +                                                                          ⁢                  16                                                                                        v                                                2                  ⁢                                      i                    1                                                  +                                                                  ⁢                24                                                                        -                              v                                                      2                    ⁢                                          i                      1                                                        +                                                                          ⁢                  24                                                                        ]      
The codebook for 8Tx is designed using a combination of Discrete Fourier Transformation (DFT) vectors and therefore is more suitable for correlated channels. In Rel 12, an enhanced design is adopted by the codebooks for the 4Tx. Specifically, the codebooks under ranks 1 to 2 are changed, and the codebooks under ranks 3 to 4 are maintained. The enhanced codebooks for the 4Tx are as shown in Table A11 and Table A12.
TABLE A11i2i101 2 3 4 5 6 70-15Wi1,0(1)Wi1,8(1)Wi1,16(1)Wi1,24(1)Wi1+8,2(1)Wi1+8,10(1)Wi1+8,18(1)Wi1+8,26(1)i2i1891011121314150-15Wi1+16,4(1)Wi1+16,12(1)Wi1+16,20(1)Wi1+16,28(1)Wi1+24,6(1)Wi1+24,14(1)Wi1+24,22(1)Wi1+24,30(1)       where    ⁢                  ⁢          W              m        ,        n                    (        1        )              =            1      2        ⁡          [                                                  v              m              ′                                                                                          φ                n                ′                            ⁢                              v                m                ′                                                        ]      
TABLE A12i2i1 0 1 2 30-15Wi1,i1,0(2)Wi1,i1,1(2)Wi1+8,i1+8,0(2)Wi1+8,i1+8,1(2)i2i1 4 5 6 70-15Wi1+16,i1+16,0(2)Wi1+16,i1+16,1(2)Wi1+24,i1+24,0(2)Wi1+24,i1+24,1(2)i2i1 8 910110-15Wi1,i1+8,0(2)Wi1,i1+8,1(2)Wi1+8,i1+16,0(2)Wi1+8,i1+16,1(2)i2i1121314150-15Wi1,i1+24,0(2)Wi1,i1+24,1(2)Wi1+8,i1+24,0(2)Wi1+8,i1+24,1(2)       where    ⁢                  ⁢          W              m        ,                  m          ′                ,        n                    (        2        )              =            1              8              ⁡          [                                                  v              m              ′                                                          v                              m                ′                            ′                                                                                          φ                n                            ⁢                              v                m                ′                                                                                        -                                  φ                  n                                            ⁢                              v                                  m                  ′                                ′                                                        ]      
The above illustrates a principle of a codebook feedback technology in the LTE. During practical application, some detailed feedback methods may be involved.
First of all, a feedback granularity of the channel information is introduced as follows. In an LTE standard, a minimum feedback unit of the channel information is subband channel information. One subband may be composed of a plurality of Resource Blocks (RBs). Each of the RBs may consist of multiple Resource Elements (REs). The RE here is considered as a minimum unit for a time-frequency resource. A resource representation method of the LTE continues to be used in the LTE-A. Several subbands may be called as a multi-subband, and a great number of subbands may be called as a wideband.
A feedback content associated with the channel information in the LTE is introduced as follows. The types of Channel State Information (CSI) fed back may include: Channel Quality Indication (CQI) information, a Program Management Instruction (PMI) and a Rank Indicator (RI). Here, a most concerned CSI content may be the PMI information, but of course, the RI and the CQI also pertain to the content in the CSI feedback.
The CQI is an indicator for evaluating whether a downlink channel is good or bad. In a 36-213 protocol, the CQI is represented by an integer value within a range of 0-15. These integer values respectively represent different CQI levels, and different CQIs may correspond to their respective Modulation and Coding Schemes (MCS).
The RI is used for indicating the number of spatial independent channels and corresponds to ranks of a channel response matrix. Under open-loop spatial multiplexing and closed-loop spatial multiplexing modes, there is a need for a User Equipment (UE) to feed back the RI information. However, under other modes, the RI information may not need to be fed back. The ranks and the layers of the channel matrix are corresponding to each other.
Along with high-speed development of wireless communication technologies, wireless applications of users are increasingly rich, which leads to rapid growth of wireless data services. In future ten years, the data services may grow at 1.6-2 times of the rate each year and in turn brings an enormous challenge to a wireless access network. A multi-antenna technology is a key technology to cope with the explosive growth of the wireless data services. At present, the multi-antenna technology supported in a 4th Generation (4G) only supports a horizontal-dimension beamforming technology for eight ports at maximum and hence there is still a relatively large potential to greatly improve the system capacity.
With the development of the communication technology, a base station side may be provided with more antennas, thereby further improving the system capacity. While the antennas are increased and particularly a Three-Dimensional (3D) channel is established, it may be needed to redesign a downlink multi-antenna codebook. The design for the 8Tx described above complies with a Group of Blocks (GoB) model and may be viewed as follows.W=W1·W2  (2)
Where W1 is composed of four adjacent DFT vectors and is indicated by i1·W2 indicates a combination of sequence numbers of columns extracted from the W1 and is indicated by i2. Due to a W1 form, the codebook for the 8Tx may be only suitable for the correlated channels. Once the W1 for a long-cycle feedback is wrongly selected, within this cycle, no matter how the W2 may be selected, a suitable codebook may not be found and therefore the performance of the whole cycle may be affected.
The above enhanced codebook for the 4Tx also employs the GoB model and is also represented by the model of the Formula (2). A difference between enhanced codebooks for the 4Tx and the 8Tx is that the W1 of the enhanced codebook for the 4Tx consists of four orthogonal nonadjacent DFT vectors and is indicated by i1. W2 indicates a combination of sequence numbers of columns extracted from the W1 and is indicated by i2. The reasons for such design lie in that when the base station side adopts 4Tx dual-polarization configuration, the distance between the antennas may be very large and even up to four times of a wavelength. Under such a condition, the channel may be uncorrelated and therefore the designed code words are suitable for the uncorrelated channels. From a composition of the codebook for the 4Tx, it may be seen that when the W1 for the long-cycle feedback is wrongly selected, the performance may not be seriously affected. However, after the W1 for the long-cycle feedback may be selected, as the four DFT vectors in the W1 are far apart, when the channel changes slowly, the same DFT vector combination may be selected by the W2 all the time, such that the code word may not reflect the channel change.